Remove source URL validation in scraper

[fbacall]

Summary of changes

• Removes the validate_url call when running a source through a Scraper.
• Ensures no external network requests are made when running tests.
• Adds a with_net_connection test method to allow real network calls in tests if needed (currently used to test private_address_check is working).

Motivation and context

A user created a source with a URL that returned a 301 redirect. This worked fine when testing, but was considered invalid when running through the scraper. The URL format is already validated when creating a source, and any issues when resolving the URL should be raised when running the actual Ingestor.

Checklist

• I have read and followed the CONTRIBUTING guide.
• I confirm that I have the authority necessary to make this contribution on behalf of its copyright owner and agree
  to license it to the TeSS codebase under the
  BSD license.